The present invention relates to breaking sheets of brittle materials using a laser-scribing technique. The present invention has a particular applicability in laser-scribing brittle non-magnetic sheets along curvilinear paths to produce substrates for use in manufacturing magnetic recording media.
Conventional techniques for cutting a sheet of brittle material, such as a glass, amorphous glass, glass-ceramic or ceramic material, comprises two principal methods. One such method is mechanical-scribing employing a hard device, such as a diamond tip, to create score marks on the surface of the brittle material which is then broken along the score marks. The second of such conventional techniques is laser-scribing, employing a continuous wave laser, such as a CO2 laser, to continuously heat a zone of a brittle material, such as amorphous-glass, similar to a floating window, up to a localized temperature below the softening point of the material, and then immediately quenching the heated body by applying a coolant, such as air or a liquid such as water. The heating-quenching combination will initiate a tiny surface crack that propagates to localize compression-tension stress effects and the sheet of material will finally be broken under external thermal or mechanical stress. A conventional laser-scribing technique is disclosed by Kondratenko in U.S. Pat. No. 5,609,284, wherein an elliptical target area is impinged with a beam of coherent radiation along the intended direction of the crack, while a stream of fluid coolant is directed at a point on the heated surface on the intended line of the crack. Allaire et al. in U.S. Pat. No. 5,776,220 disclose a laser-scribing technique for brittle materials wherein the laser spot has an extremely elongated elliptical shape such that its major axis is greater than 20 mm to enable rapid scribing.
Conventional substrates for use in manufacturing magnetic recording media include various brittle materials, such as glasses, ceramics and glass-ceramics. In order to form a suitable substrate for use in a magnetic recording medium, two circular scribings must be performed with high precision, one defining the outer diameter and one defining the inner diameter. However, current linear laser-scribing techniques for flat panels can not be directly applied in circular scribings for producing magnetic recording media. For flat panel linear laser-scribing, a circular laser light beam is typically reshaped into a beam with a very elongated cross section, as shown in FIG. 1, with a short axis xe2x80x9caxe2x80x9d of about 1xc2x10.5 mm, and a long axis xe2x80x9cbxe2x80x9d of about 30xc2x110 mm. In FIG. 1, xe2x80x9cxxe2x80x9d represents the scribing direction, xe2x80x9cHxe2x80x9d represents the scribing heating zone, xe2x80x9cQxe2x80x9d represents the quenching zone, xe2x80x9cTspxe2x80x9d represents the material softening point, xe2x80x9cPHxe2x80x9d represents the preheating zone, xe2x80x9cLSxe2x80x9d represents the laser-scribing heating temperature and xe2x80x9cQTxe2x80x9d represents the quenching temperature.
Adverting to FIG. 1, the localized temperature increases from the leading point to the central point where the highest temperature is achieved but below the material""s softening point. The purpose for a long beam axis along the scribing direction and raising a temperature profile is to provide preheating of the surface before reaching the scribing/quenching point. Such an elongated beam is not, however, suitable for substantially circular scribing. Although it may be theoretically possible to modify the lens optics to reshape the elongated beam into an elongated curved beam, any such optical design would be extremely complex in order to meet the various requirements of the outer diameter and inner diameter of magnetic recording media. For example, typical magnetic recording media have an inner diameter of about 20 mm to about 25 mm and an outer diameter ranging from 65 mm to 95 mm, e.g., 84 mm. The difficulty in laser-scribing along a curvilinear path is recognized by Kondratenko (in U.S. Pat. No. 5,609,284) who suggests the use of a score or nick having a gradually increasing depth along the cutting line.
Accordingly, conventional practices for cutting brittle substrates for use in magnetic recording media involve the use of diamond tips to induce mechanical stresses. However, the required high tolerances are not fully satisfied by mechanical scribing. Moreover, after mechanical scribing it is necessary to perform extensive polishing and lapping, thereby significantly increasing manufacturing costs.
There exists a need for a method and apparatus for laser-scribing a brittle substrate along a curvilinear path, particularly a substantially circular path. There exists a particular need for an apparatus and methodology for laser-scribing brittle materials, such as glasses, ceramics and glass-ceramics, along substantially circular paths to form substrates for use in manufacturing magnetic recording media.
An advantage of the present invention is a method of laser-scribing brittle materials along a curvilinear path.
Another advantage of the present invention is an apparatus for laser-scribing brittle materials along a curvilinear path.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following disclosure or may learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
Accordingly to the present invention, the foregoing and other advantages are achieved in part by a method of laser-scribing to separate portions of a sheet of brittle material along a curvilinear path, the method comprising: rotating the sheet; impinging a first laser light beam at a first beam energy on a surface of the sheet to form a first laser spot having a first size to preheat the surface of the sheet to a first temperature; and impinging a second laser light beam at a second beam energy on the surface of the sheet to form a second laser spot having a second size, spaced apart from and behind the first laser spot with respect to the rotating direction, to heat the surface to a second laser-scribing temperature higher than the first temperature; and directing a coolant to an area on the surface of the sheet behind the second spot with respect to the rotating direction.
Another aspect of the present invention is an apparatus for laser-scribing a sheet of brittle material into separate portions along a curvilinear path, the apparatus comprising: means for rotating the sheet; a laser source; a coolant source; means for: impinging a first laser light beam at a first beam energy on a surface of the sheet to form a laser spot having a first size to preheat the surface of the sheet to a first temperature; and impinging a second laser light beam at a second beam energy on the surface of the sheet to form a second laser spot, behind the first laser spot with respect to the rotating direction, to heat the surface of the sheet to a second laser-scribing temperature greater than the first temperature; and means for directing a coolant to an area on the surface of the substrate behind the second spot with respect to the rotating direction.
A further aspect of the present invention is a laser-scribing apparatus for separating portions of a sheet of brittle material along a curvilinear path, the apparatus comprising: a source of a coolant; and means for heating a surface of the sheet to a first preheating temperature and to a second laser-scribing temperature greater than the first preheating temperature.
Embodiments of the present invention comprise the use of a crystal material to form first and second laser light beams. In another embodiment of the present invention, first and second laser light beams are formed employing first and second fiber optic cables each optically linked to a lens. In another embodiment, a three-spot defractive lens is employed to form first and second laser light beams in addition to a third laser light beam which impinges the surface to form a third spot between the second spot and the area impinged by the coolant. Embodiments of the present invention include laser-scribing a glass, ceramic or glass-ceramic material to form a substantially circular disk having an outer diameter and an inner diameter for use in manufacturing magnetic recording media. It should be recognized that the embodiments disclosed herein can be employed for scribing straight lines as well as free-shape lines, such as curvilinear lines and substantially circular lines.
Additional advantages of the present invention will become readily apparent to those having ordinary skill in the art from the following detailed description, wherein embodiments of the present invention are described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.